The present invention relates to vertical magnetization type recording media and methods for manufacturing such media. More particularly, the invention relates to a vertical magnetization type recording medium of a type having a support with a soft magnetic film layer and a vertical magnetization film layer formed on each side of the support, and to a method of manufacturing such a recording medium.
Recently, a vertical magnetization type recording system has been proposed which uses a magnetic recording medium having an "easy" magnetizing axis perpendicular to its film surface. In the vertical magnetization recording system, as the recording density increases, the diamagnetic field in the recording medium is reduced, and therefore an excellent reproduction output is provided. Thus, the system is fundamentally suitable for high-density recording.
In order to perform a magnetic recording operation using the vertical magnetization type recording system, it is necessary to employ a magnetic recording medium having an easy magnetizing axis perpendicular to the film surface. An example of a suitable vertical magnetization type recording medium is one provided by forming a Co-Cr film layer on a support of macromolecular material or non-magnetic material such as a non-magnetic metal by sputtering or the like.
For improving the recording and reproducing efficiency for vertical magnetization type recording and reproducing, a so-called "two-film-layer vertical magnetization type recording medium" has been proposed in which a high-magnetic-permeability layer of soft magnetic material such as a permalloy (Ni-Fe alloy) film layer is formed, as a base layer, under the vertical magnetization type recording layer, namely, the aforementioned Co-Cr film layer.
For a flexible disk according to the aforementioned vertical magnetization type recording system, a so-called two-layer vertical-magnetization type recording medium, on both sides of which the two-layer vertical magnetization type recording layer is formed, is suitable because it provides a large recording capacity and curling is not a problem.
Heretofore, the above-described double-sided two-layer vertical magnetization type recording medium has been manufactured using a continuously running film-shaped support with a double-sided sputtering apparatus as shown in FIG. 1.
A method is known in which a two-film layer is formed on each side of the film-shaped support with the apparatus shown in FIG. 1 while the support is run in only one direction.
The apparatus has a sputtering chamber 40 in which cylindrical cans 23 and 24 are arranged. After the sputtering chamber is evacuated, argon gas is introduced thereinto in an amount so that the pressure in the chamber is maintained at about 3.times.10.sup.-3 torr. A PET film is supplied from a supply shaft 22 in the chamber 40. At the can 23, a permalloy film layer is formed on one side of the PET film thus supplied by sputtering using a permalloy sputtering source 31, and a Co-Cr vertical magnetization film layer is formed on the permalloy film layer thus formed using a Co-Cr sputtering source 32. In succession, at the can 24, a permalloy film layer is formed on the other side of the PET film using a permalloy sputtering source 33, and a Co-Cr vertical magnetization film layer is formed on the permalloy film layer using a Co-Cr sputtering source 33. The PET film is wound on a take-up shaft 25 after being treated as described above. That is, in this method, a two-film layer is formed on one side of a film-shaped support by sputtering using the permalloy target 31 and the Co-Cr target 32, and then another two-film layer is formed on the other side with using the permalloy target 33 and the Co-Cr target 34.
In the case where a double-sided two-layer vertical magnetization type recording medium having Co-Cr film layers of high coercive force H.sub.c (vertical) in a vertical direction is formed employing a high-speed sputtering method such as a DC magnetron sputtering method, it is generally desirable that the temperature of the support be high during the sputter deposition of the Co-Cr film layer.
However, in the double-sided two-layer vertical magnetization type recording medium manufactured using a film-shaped support of macromolecular material as described above, when the support is heated, the surface characteristics of the support tend to be deteriorated, the vertical orientation of the vertical magnetization film layer disturbed, and both sides of the recording medium tend to have different surface characteristics, vertical orientation and H.sub.c (vertical), with the result that the recording and reproducing characteristics are poorer than desired. Thus, the conventional process involves serious problems in the mass production of the recording medium.
In the case where the double-sided two-layer vertical magnetization type recording medium is formed using the winding type continuous sputtering apparatus shown in FIG. 1, in general, tension is applied in one direction to the film-shaped support. As a result, the static magnetic characteristics of the soft magnetic film layer tend to be anisotropic; that is, the film layer tends to become magnetically anisotropic, and the static magnetic characteristics, for instance, the magnetic permeability, tend to vary in different directions. Therefore, when a recording and reproducing operation is carried out with a floppy disk formed by stamping (cutting) such a recording medium, the reproduction output often varies in the circumferential direction.
Intensive research has been carried out by the present inventors to overcome the above-described difficulties, and it was found that the static magnetic characteristic of the soft magnetic film layer can be made substantially isotropic by heating the cylindrical can to 30.degree. C. to 90.degree. C. during the sputtering of the soft magnetic film layer.
In addition, it was found that if both sides of the film-shaped support are covered with a soft magnetic film layer before the vertical magnetization film layer, such as a Co-Cr film layer, is formed thereon, the discharge of impurity gas and the deposition of oligomer from the film-shaped support of macromolecular material or the like is suppressed. Therefore, it becomes possible to heat the cylindrical cans to 90.degree. C. or higher in forming the Co-Cr film layers by sputtering, thereby to form a double-sided two-layer vertical magnetization type recording medium which has vertical magnetization film layers of high coercive force H.sub.c (vertical) and excellent vertical orientation and of which the two sides have uniform characteristics.
According to this method, a double-sided two-layer vertical magnetization type recording medium is obtained which has less reproduction output variation in the circumferential direction than the conventionally produced medium, and which has excellent vertical magnetization characteristics. However, it has been found that the recording medium is still disadvantageous in that the soft magnetic film layer on each side of the recording medium and the vertical magnetization film layer formed on the soft magnetic film layer have relatively poor surface characteristics; that is, the surfaces of these film layers are relatively rough. Furthermore, the characteristics of the soft magnetic film layer formed on the side of the film-shaped support tend to be somewhat different from those of the soft magnetic film layer formed on the other side.
Moreover, probably because of the use of the cylindrical cans, in the recording medium manufactured according to the conventional method, the magnetic film and the film-shaped support are strained in different manners. As a result, the recording medium generally curled a great deal. A strongly curled recording medium is not practical in use.
On the other hand, a vertical magnetization type recording medium which scarcely curls has been proposed in Japanese Patent Application Publication No. 2230/1984. This recording medium is a double-sided single-layer vertical magnetization type recording medium in which curl balance is effected by adjusting the thicknesses of the vertical magnetization film layers formed on the two sides of the film-shaped support.
However, this conventional vertical magnetization type recording medium is disadvantageous in that its two sides do not have the same electromagnetic conversion characteristics. For instance, if the vertical magnetization film layer on one side of the recording medium has a greater thickness than that of the other side, the one side provides a greater low-frequency reproduction output than the other side. Furthermore, the double-sided single-layer vertical magnetization type recording medium provides a lower reproduction output than the double-sided plural-layer vertical magnetization type recording medium.
Accordingly, the inventors have conducted intensive research to find a double-sided plural-layer vertical magnetization type recording medium which scarcely curls and which can provide substantially-equal high-level reproduction outputs from both its sides.
In an attempt to do this, the inventors have tried recording media in which the magnetization film on one side of a double-sided plural-layer vertical magnetization type recording medium is made different from that of the vertical magnetization film on the other side. It was found that the recording media scarcely curled, but the reproduction output from the two sides thereof differed greatly.
In addition, it has been found that, if the thickness of the soft magnetic film layer on one side of the recording medium is made different from that of the soft magnetic film layer on the other side, the recording medium scarcely curls and the reproduction outputs of both sides are substantially equal. However, the reproduction characteristics of the recording media tend to differ greatly among lots, and if the thickness differs from the optimum value even slightly, the recording medium has a strong tendency to curl; that is, the manufacture of the recording medium requires severe manufacturing tolerances. Moreover, in order to obtain a satisfactory curl balance, it is necessary to provide a relatively large difference in thickness between the two soft magnetic film layers, which makes the two sides of the recording medium have different durabilities.